how often do you sharpen blades

how often do you sharpen your blades and how many sets do you have? what do you use to sharpen them?
i sharpen mine every 2-6 lawns depending on size.
i have 2 sets for each mower.
i use a hand grinder to sharpen them.

geeeez, i rarely change my blades that often, i mow like 30 a week on my mower, and i change them every 1.5 - 2 months. I guess you guys have small pebbles for turf, but my blades are in grade condition and the grass looks perfect after im done:-\ im cutting st augustine and bermuda, am i missing something here?